How wide should the rod be?
Run the rod 8"–12" past the window trim on each side. When the curtains are open, the panels stack on the wall instead of over the glass, so you keep all the daylight and the window looks wider. A 48" window with 10" on each side takes a 68" rod — buy an adjustable rod that reaches at least that far.
How high should the rod go?
The usual spot is 4"–6" above the top of the window trim. To make a window look taller, hang the rod about halfway between the trim and the ceiling, or just under the crown molding. With less than 2" above the rod, switch to ceiling brackets or a track.
How many panels?
Multiply the rod width by the fullness you want and divide by the panel width. 2x fullness looks full and gathered and is the safe choice; 1.5x looks flat and tailored; 2.5x–3x looks luxurious, especially with sheers. A 68" rod at 2x fullness needs 136" of fabric — three 54" panels in theory, so buy four, two on each side.
What length to buy?
Measure from the top of the rod to the floor. Curtains that float about ½" off the floor are easiest to live with. A break of 1"–2" looks relaxed, and a puddle of about 6" looks romantic but collects dust. Stock lengths are 63", 84", 95", 96", 108" and 120". If your number falls between two, buy the longer panel and hem it, or move the rod up an inch or two.
Grommets, rings and clips
Grommet and rod-pocket panels hang from the top of the rod, so measure from there. Rings and clips drop the panel by about 1"–2"; measure from the bottom of the ring instead.
Brackets
Rods up to about 60" need a bracket at each end. Longer rods need a center support, or they sag. Screw brackets into a stud or use wall anchors rated for the weight of the curtains.
Planning the whole room? Size the floor covering with the rug size calculator.